As is well known, thin metal lids are produced by blanking operations from thin metal sheets which are normally already painted or varnished. In many applications, particularly in the food-preservation field, "easy-to-open" metal cans are used. This term generally means metal cans the lids of which normally have a pull tab and a predetermined break line, and which can be opened without the need for additional tools. In order to create the predetermined break line, the already-painted or varnished lids are cut on the surface which is to be the outer surface; this operation consequently creates a cracking or breakage of the film of paint or varnish on the cut portions, which are thus exposed.
To prevent corrosion due to the pasteurization processes as well as simply due to atmospheric agents, the exposed portions of the cut lids have to be re-painted or re-varnished.
Two techniques are currently known for this painting or varnishing step; the first uses painting or varnishing by atomization, and the second uses painting or varnishing by electrophoresis. The second technique is considered without doubt the best since it ensures that all of the uncovered metal parts are fully painted or varnished.
A known machine for the electrophoretic painting or varnishing of thin metal lids uses, as an anode unit, a thin, flexible element which bears on the rims of the edges of the lids to be painted or varnished which have been exposed by the blanking operation. The lids to be painted or varnished travel in a horizonal position on a conveyor line for the lids and the flexible elements are moved at the same speed, maintaining contact with the lids throughout the period during which they pass through the electrophoretic bath. The anode unit, which consists of a wire or metal strip, follows an endless path with respective guides and drive pulleys.
However, the machine just described has a fairly important disadvantage; the wire constituting the anode is immersed in the electrophoretic bath, and is therefore covered with a large amount of paint or varnish which cannot be removed and which irreparably soils the guides and the drive means for the wire. Moreover, the wire constituting the anode is under tension along the entire endless circuit, causing well-known problems of premature wear of the guide and return elements owing to electrical erosion.
A further disadvantage of this machine is the system for transporting the lids. In fact it must be borne in mind that the transportation of the lids in a horizontal position prevents perfect painting or varnishing since the oxygen which forms in the vicinity of the lower surface of the lid during anaphoresis remains incorporated under the lid, in addition to any air already trapped by the bell effect of the concave side of the lid which faces downwardly at the moment of immersion, thus creating an insulating cushion which partially prevents the deposition of paint or varnish on the metal object.
Moreover, since the conveyor line for the lids necessarily has to be horizontal, the tank for the electrophoretic bath has to have two openings for allowing the conveyor line for the lids to enter and leave the tank, respectively. Clearly, these two openings have to be below the level of the electrophoretic bath, thus causing a continual leakage of liquid. The machine described above therefore requires a system for circulating the liquid which has leaked from the electrophoretic tank, thus increasing the structural complexity of the machine.
To prevent the problem of the deposition of paint or varnish on guides and pulleys caused by the wire constituting the anode of the machine described above, a further known machine for the electrophoretic painting or varnishing of thin metal objects, substantially similar to that just described, comprises an anode system constituted by a metal conveyor chain having a plurality of appendages each of which is intended to contact a respective lid to be polarized. Only the appendages of the anode system are therefore immersed in the electrophoretic bath, preventing paint or varnish from being carried along by the wire.
With the construction of the machine just described, however, the other problems mentioned above still remain unsolved, that is, wear due to electrical erosion, the painting or varnishing problems connected with the horizontal transportation of the lids, and the need for the machine to be rendered more complex by a system for recycling the paint or varnish which continually leaks from the electrophoretic tank.